1. Field of the Invention
The present invention relates to an organic light emitting diode (OLED) display device and a method of fabricating the same.
2. Description of the Related Art
Generally, an organic light emitting diode (OLED) display device is an emitting display device, in which an electron and a hole are respectively inserted from an electron injecting electrode (cathode) and a hole injecting electrode (anode) into an emission layer, and the injected electron and hole are combined to thereby create an exciton, which emits light when transitioning from an excited state to a ground state.
Unlike a conventional liquid crystal thin film display device, a separate light source is not required due to the above structure, and this results in reduced size and weight of a device.
Generally, an OLED display device includes a substrate, a lower electrode disposed on the substrate, an organic layer including an emission layer disposed on the lower electrode, and an upper electrode disposed on the organic layer. The organic layer may further include a hole injection layer (HIL), a hole transport layer (HTL), and an electron blocking layer (EBL) between the lower electrode and the emission layer, and a hole blocking layer (HBL), an electron transport layer (ETL), and an electron injection layer (EIL) between the emission layer and the upper electrode.
Also, the OLED display device may be classified into a bottom-emitting type and a top-emitting type depending on an emission direction of light generated from the emission layer. In the bottom-emitting type, light emits towards the substrate, an upper electrode is formed of a reflective electrode, and a lower electrode is formed of a transparent electrode. Here, when the OLED display device adopts an active matrix type, in which a thin film transistor is formed, light cannot pass through a region where the thin film transistor is formed, and thus an area through which light can pass is reduced. In contrast, in the top-emitting type, an upper electrode is formed of a semi-transmissive metal layer, and a lower electrode is formed of a transparent electrode including a reflective layer, so that light emits towards a direction opposite to a substrate, and an area through which light passes becomes larger than that of the bottom-emitting type.
In a conventional top-emitting OLED display device, an upper electrode is formed of a transparent conductive material such as Indium Tin Oxide (ITO) or Indium Zinc Oxide (IZO) or a very thin metal layer such as magnesium-silver (MgAg) so that light passes through the upper electrode.
However, when the upper electrode is formed of the transparent conductive material such as ITO or IZO, the upper electrode may have a high transmittance, but it is not appropriate due to a high work function when an upper electrode is a cathode. Also, when the upper electrode is formed of the semi-transmissive metal layer such as MgAg, the semi-transmissive metal layer should be formed thin in order to enhance transmittance, so that it has a problem of an increased resistance value.